1. Field of Invention
This invention relates generally to a brushless servo motor tester and specifically to a brushless servo motor tester that tests a motor in its operating environment.
2. Description of Prior Art
Industrial automation applications often use permanent magnet brushless (PMBL) servo motors for positioning. Applications are often complex, three-dimensional positioning along with rotary alignment requiring multiple axes of control, operating cooperatively, to accomplish a task.
A typical PMBL servo motor comprises an armature with armature windings, a rotor, and rotor state feedback. The rotor is connected to, and is rotatably supported by, a motor shaft, on an axis of rotation. The rotor angle is a mechanical angle representing shaft rotation. The rotor comprises one or more permanent magnet pole pairs. The armature windings, when powered, produce an armature magnetic field that interacts with the rotor field to produce rotor torque. The armature windings are disposed about the circumference of the rotor. PMBL servo motors typically have three armature windings or phases, but must have at least two.
PMBL servo motors require commutation of the power applied to the armature windings to operate. Commutation switches the DC power applied to the armature windings into a polarity sequence that generates a rotating armature magnetic field. The armature field rotates about the rotor axis. Commutation of armature power is synchronized to the rotor angle. FIG. 1 depicts a typical control axis. The inner loop of the control axis comprises PMBL servo motor 10 and drive system 20. Electronics in drive system 20 produce drive signals 30 to power the armature windings to move the rotor. Drive system 20 synchronizes armature drive signals 30 to the rotor angle by means of rotor state feedback 40. Rotor state feedback comprises rotor angle and, depending on the motor, may include velocity and/or accumulated rotor angle. Generally, the commutation sequence of drive signals 30 controls the direction of motor rotation, while drive voltage level determines motor rotation speed. Rotor torque increases with armature current. The inner control loop is responsible for commutation of power supplied to the armature windings along with speed and direction control.
The armature magnetic field angle, also called the electrical angle, rotates N times per shaft revolution for a 2N-pole rotor. Drive system 20 generally produces armature drive at the electrical angle that produces maximum rotor torque at the current rotor angle.
The outer control loop comprises controller 60 in addition to the inner control loop. The outer control loop is responsible for coordination of the local control axis with the larger system requirements. Sensor inputs 70, control inputs 80, and rotor state feedback 40 are used to determine the required speed and direction input 50 to drive system 20.
PMBL servo motors generally have three phase armature windings comprising three windings. The three windings are generally interconnected either in a wye or delta configuration. In the wye configuration, one end of each of the three windings is connected together, leaving three terminals to power the armature. In the delta connection, the windings form the three legs of a triangle, with armature power applied to the terminals formed at the three vertices of the triangle. In both cases, applying power between terminals powers multiple windings.
Two types of PMBL servo motors are in use, DC and AC. AC and DC motors are similar in construction. DC motors generally use six-state commutation that advances the electrical angle in sixty-degree steps. DC motors often use a Hall effect angle encoder to produce rotor state feedback. Six-state angle encoders provide sixty-degree angle resolution of the electrical angle. FIG. 2 shows typical armature drive polarity sequence for one revolution of a motor with a two-pole rotor. The electrical and mechanical angles of a two-pole motor rotate at the same rate. The armature excitation is a DC voltage applied across two terminals at a time at a constant voltage (for a given speed). A given pair of armature terminals is powered for 60-degrees of rotor rotation before the armature excitation is moved to a different terminal pair.
PMBL AC servo motors have some advantages over PMBL DC servo motors in providing finer positioning capability and smoother torque. These advantages come at the cost of more complex drive requirements in drive system 20 that must provide three-phase sinusoidal drive to the motor armature windings. Sinusoidal armature drive in turn requires higher resolution rotor angle feedback. Applying DC servo motor armature drive signals to an AC servo motor will drive the rotor in accordance with FIG. 2 as with a DC servo motor. Servo motor design provides optimum performance with matching armature drive, however, both AC and DC servo motors will rotate in response to DC armature drive.
Because of the complexity of motion control system 90, including motor 10, drive system 20 and controller 60, when the system stops working, it is difficult to determine which component failed. Feedback systems are often difficult to diagnose and systems with multiple feedback loops, such as motion control system 90, compound the problem. Drive system 20 and controller 60 often perform fault checks and shut down when a fault is detected. Because the system is not moving, suspicion often falls on motor 10, which is then removed and replaced. Because these motors are often embedded in complex machine tools, costly downtime ensues and oftentimes the problem remains after motor replacement.
Several items must be identified in order to successfully test a motor. These include the motor operating parameters: rated current and the number of rotor poles, as well as the motor configuration: the feedback device types and related parameters. Motors are often not directly labeled with enough information to directly determine the operating parameters and configuration. The step of identifying the operating parameters and configuration from the motor part number is time consuming and error prone.
The prior art does not sufficiently address in situ servo motor testing. Accordingly, there exists a need for diagnostic equipment and methods to evaluate PMBL servo motors for failure prior to removal from service.
Objects and Advantages
It is therefore an object of the present invention to provide an apparatus and method for diagnosing servo motor operational status prior to removal.
A further object of the present invention is to test for proper servo motor rotor state feedback operation.
A further object of the present invention is to test for proper servo motor armature operation prior to removal.
It is a further object of the present invention to simplify the determination of motor configuration and operating parameters.
It is still another object of the present invention to automatically identify the motor configuration and operating parameters.